Tertiary oils from Upper Assam Basin, India: A geochemical study using terrigenous biomarkers

The Upper Assam Basin in northeast India is a Tertiary sub-basin of the Assam-Arakan Basin. Hydrocarbons are present in the Langpar Formation (Upper Palaeocene), Lakadong member of the Sylhet Formation (Lower Eocene), and the Barail (Oligocene), Tipam (Lower Miocene) and Girujan (Middle Miocene) formations in the basin. A detailed study to understand the nature and origin of the oils from different stratigraphic horizons using terrigenous and other biomarkers has been carried out. Further, the variation in maturities of oils occurring in different stratigraphic horizons has also been studied. The study shows that rearranged oleananes, oleananes and bicadinanes are present in these oils implying land plant input to their source rocks. The oils have been classified into different groups, using ratios of rearranged oleananes, oleananes, bicadinanes and steranes, indicating presence of multiple source rocks in the basin with subtle variation in characteristics. Maturity studies show that oils from younger horizons (Barail, Tipam and Girujan) are more mature than oils from older horizons (Langpar and Lakadong). There is an apparent mismatch in maturity of oils as measured by different biomarker maturity parameters. This is due to the mixing of two different charges of oils, having different maturities and generated from two different source rock kitchens, in the same reservoir. Principal component analysis (PCA) of biomarker data has also been able to classify the oils into various groups. A new parameter has been developed, using ratios of rearranged oleananes, for determining maturity of these oils.